1. Technical Field
The present invention relates to a base for a fixing belt employed in a copier, a printer, or a facsimile machine employing electrophotography, and further to a fixing belt, a fixing device, and an image forming apparatus incorporating such a base for the fixing belt.
2. Related Art
In an image forming apparatus employing electrophotography, such as a copier, a printer, and a facsimile machine, a roller or a belt having a base layer of seamless, nickel-electroformed film is widely used as a heating and fixing member for fixing toner.
Herein, an example of a conventional toner fixing method will be described.
FIG. 1 illustrates an image forming apparatus; FIG. 2 illustrates a fixing device used in the image forming apparatus of FIG. 1; and FIG. 3 illustrates a cross-sectional view of a fixing belt used in the fixing device of FIG. 2, each of which is represented as a typical model.
As illustrated in FIG. 1, laser beams 13 are used to expose a photosensitive layer (which is previously charged by a charger) of a drum-shaped image carrier or a photoreceptor 11, based on image data, so that an electrostatic latent image is formed on the photoreceptor 11. In this case, the laser beams 13 are polarized periodically using a polygonal mirror which rotates at a predetermined speed so that the photosensitive layer of the image carrier 11 is scanned and exposed repeatedly in a main scanning direction perpendicular to a sub-scanning direction. In the present example, a roller-shaped image carrier is used; however, alternatively, a belt-shaped image carrier stretched around rollers may also be used. In this case, a transfer nip is formed between the belt-shaped image carrier and a transfer roller 15 at a portion where the belt-shaped image carrier is stretched around the roller-shaped rotary member.
Next, the electrostatic latent image thus formed on the photosensitive layer of the image carrier 11 is rendered visible by particulate toner supplied from the developing device 14 via a developing roller 14a, and thus, the toner image is formed. Thereafter, a transfer bias voltage having a polarity opposite that of the toner is applied to the transfer roller 15 from a transfer bias power supply 30. With this transfer bias voltage, the toner image is transferred to a transfer medium P that is conveyed from a sheet feeder via a conveyance roller pair 20, 21 to the transfer nip formed between the transfer roller 15 of the transfer device and the image carrier 11. Then, the toner image on the transfer medium P is pressed and fixed with a previously adjusted temperature by a fixing device 24, and the transfer medium P having the fixed image thereon is discharged to a paper discharge tray, not shown.
As illustrated in FIG. 2, the fixing device 24 includes a cylindrical or substantially cylindrical heat pipe 2 formed of thin aluminum. The heat pipe 2 includes a built-in heat generation member 1 such as a halogen heater in its center thereof. A pressure pad 4 is disposed inside the heat pipe 2. The pressure pad 4 is fixed on a stay 3 disposed inside the heat pipe 2. A seamless, electroformed nickel fixing belt 5 is mounted on a circumferential surface of the heat pipe 2. The fixing belt 5 is formed of a slidable layer, an elastic layer, and a release layer in that order from an inner side to an outer side. The heat pipe 2 is disposed opposite a pressure roller 6 via the fixing belt 5 in between, with the pressure pad 4 pressing against the fixing belt 5 from an interior side of the belt 5 to thus contact the fixing belt 5 against the pressure roller 6. The pressure pad 4 may be configured to be biased by a biasing device, not shown, toward the pressure roller 6. Alternatively, the pressure roller 6 may be biased by the biasing device, not shown, toward the pressure pad 4. Thus, a nip portion is formed between the fixing belt 5 and the pressure roller 6. In the fixing device 24, the fixing belt 5 is driven to rotate by the rotation of the pressure roller 6. When a transfer medium 7 on which a toner image is formed is supplied to the nip portion, the transfer medium 7 passes through the nip portion while being pressed and heated, and thus, the toner image is fixed thereon.
FIG. 3 illustrates an exemplary model of the fixing belt 5. A base of the fixing belt is formed of an electroformed nickel layer 51. A copper layer is laminated on the nickel layer 51, thereby improving heat conductivity.
A slidable layer 54 is laminated on an inner circumferential side of the endless belt-shaped base 51. The slidable layer 54 is formed of heat-resistant resins, such as polyimide (PI), and a copolymer of tetrafluoroethylene-perfluoroalkyl vinylether (PFA). Further, on an outer surface of the base 51, an elastic layer 52 formed of silicon rubber, and a release layer 53 formed of fluorine resins such as a copolymer of tetrafluoroethylene-perfluoroalkyl vinylether (PFA) are laminated onto the base 51, in that order.
The base for the fixing belt is formed by nickel electroforming as follows. First, a stainless, cylindrical master block the surface of which is polished and cleaned is soaked in a nickel electroforming bath and an electric current is applied to the bath so that nickel is precipitated on the surface of the master block. The cylindrical master block is taken out of the bath and the precipitated nickel electroformed film is de-molded from the master block. Upper and lower ends are cut to obtain a proper length.
The fixing belt including a metal base layer is employed for the fixing device, an image forming apparatus employs such a fixing device, and a high speed print capability is at all times required for the image forming apparatus.
However, a base for the fixing belt is not always suitable for high speed printing due to a lack of durability. Specifically, due to perpetual demand for ever-higher speed, the fixing belt is driven at a higher speed than in the conventional art, is subjected to higher pressure at a nip, and is repeatedly deformed in a shorter time period, causing cracks due to metal fatigue.
In response to the demand for higher speed, JP-2010-217347-A proposes a fixing belt formed of a base from an inner side including stainless steel, copper, and stainless steel laminated in that order. The belt formed of laminated stainless steel and copper is manufactured by a plastic molding process such as metallic rolling. Compared to electroformation, the plastic molding is inferior in terms of evenness of the thickness and moreover warps due to uneven processing remain, so that the durability is poor.
JP-2004-183034-A discloses use of electroformed nickel film as the base for the fixing belt with its crystal orientations, of which the crystal orientation ratio I(200)/I(111) is 80 or higher but 250 or lower and contains 0.03 to 0.10 mass % carbon. The same discloses that such nickel crystal orientation ratio contributes to durability. However, because nickel has a low heat conductivity, if nickel alone is used for the fixing belt, uneven heat conductivity is generated in the axial direction, which may cause a problem of defective image formation in high speed printing.
JP-2006-84718-A discloses a technique in which a cylindrical seamless nickel belt is manufactured by electroforming process by immersing a cylindrical metal master in an electrolytic solution containing not only nickel but also 10-10,000 ppm by volume fraction of at least one metal element selected from groups I, VI, VII and VIII of the Periodic Table. Nickel crystal orientation ratio I(200)/I(111) is set to ≧5.0. The same relates to an organic photoreceptor and does not consider heat conductivity. However, if such a material is used for the base of the fixing belt, unevenness of the heat in the axial direction will be caused.